1. Field of the Invention
The present invention relates generally to an image forming apparatus, and more particularly, to an image forming apparatus for developing an electrostatic latent image formed on a photosensitive drum, and for transferring the developed image onto an image recording medium such as paper.
2. Description of the Related Art
Conventional image forming apparatus include electrophotographic devices, electrostatic printers, etc. In the case of conventional image forming apparatuses, an electrostatic latent image is formed on a photosensitive drum. Toners are then electrostatically adhered to the latent image as developing agents, so that a toner image corresponding to the latent image is developed.
Subsequently, the toner image is transferred on an appropriate image recording medium, such as a copy sheet. After the completion of the image transferring, the electrostatic latent image and residual toner particles remain on the photosensitive drum. The residual toner particles are removed from the drum by a cleaning device. The electrostatic latent image is then removed from the photosensitive drum by a discharging device.
Recently, it has been demanded to reduce the size of the image forming apparatus. In this connection, Japanese Patent Disclosure No. Sho 47-11538 discloses an image forming apparatus having a reduced size by using a photosensitive drum having a reduced diameter and a device which serves as an image developing device and a cleaning device.
In the prior art, a photosensitive drum rotates twice in an image forming cycle. An image developing process is carried out by the device in the first rotation of the photosensitive drum, while a cleaning process is carried out by the device in the second rotation of the photosensitive drum.
The prior art devices, however, have some problems. That is, the image forming speed is halved, in comparison to conventional devices, because the photosensitive drum must rotate twice in the one image forming cycle. Further, the size of image recording media, i.e., copy sheets, is limited to a relatively small sheet size, because the length of the copy sheets available for the apparatus is required to be less than the peripheral length of the photosensitive drum and the photosensitive drum, as explained above, has a reduced diameter.
U.S. Pat. No. 4,727,395 discloses an image forming apparatus having a device which carries out concurrently the image developing process and the cleaning process. The image forming cycle of the apparatus is performed within one rotation of its photosensitive drum. Thus, the latter prior art has reduced the size of the apparatus without lowering the image forming speed.
The latter prior art, however, has another problem. That is, in the latter prior art, the residual latent image and the residual toner image still remain in the next image forming cycle. The charging process, the latent image forming process and the developing process in the next cycle are carried out on the residual latent image and the residual toner image. Thus, a resulting image formed in the next cycle is deteriorated by the residual images remaining from the preceding cycle. This kind of image deterioration becomes noticeable when a so-called solid area of the resultant image (i.e., a resultant toner image having a wide area) overlaps a residual latent image. Moreover, the residual toner image also appears on the resultant image and deteriorates the image.
Thus, the prior art image forming apparatus fail to produce satisfactory distinct images. Also, the making of the photosensitive drum smaller than the size of the image recording media has not been achieved.